1. Field of the Invention
The present invention relates to a light guide, an illumination device, and an image reading apparatus. In particular, the present invention is suitable for an image reading apparatus, such as an image scanner, a copying machine, and a facsimile machine, for reading an image by a line sequential method through illumination of a surface of an original.
2. Description of the Related Art
Conventionally, as an illumination device to be used for an image reading apparatus, such as an image scanner, a copying machine, and a facsimile machine, for reading an image by sequentially and linearly illuminating a surface of an original, a tubular (linear) light source such as a fluorescent lamp has been used. As a result of recent technology development, luminous efficacy of a light emitting diode (LED) has been enhanced, and there has been developed a technology in which the LED is used as the light source to serve as the illumination device. In a case where a point light source such as an LED is used as the light source, multiple point light sources are arrayed in a predetermined direction (main scanning direction).
In this case, when a reading surface such as a surface of an original is directly irradiated with light beams emitted from the multiple point light sources, illuminance unevenness occurs in the main scanning direction, and density unevenness occurs in the image obtained through the reading. Therefore, in the field of the image reading apparatus using multiple point light sources, there have been made inventions intended to reduce the illuminance unevenness in the light source array direction (main scanning direction).
Japanese Patent Application Laid-Open No. 2005-156600 discloses that light diffusing parts are provided at regular intervals along the light source array direction (main scanning direction) so as to diffuse light beams randomly in a range from the light sources to a subject (original), to thereby eliminate the illuminance unevenness. Specifically, each light diffusing part is formed by surface roughening, matte coating, or texturing.
Japanese Patent Application Laid-open No. 2009-80173 discloses that cylinder surfaces each having a positive power (refractive power) in the main scanning direction are periodically provided in a reflecting part of the light guide, to thereby suppress the illuminance unevenness in the main scanning direction.
However, in Japanese Patent Application Laid-Open No. 2005-156600, similarly to a center of the light guide, the light beams are diffused randomly at its edges in the main scanning direction, and hence a large amount of light beams is diffused also in a region other than a predetermined region to be illuminated. Therefore, light use efficiency of the light beams emitted from the light sources becomes lower.
In Japanese Patent Application Laid-Open No. 2009-80173, the cylinder surfaces uniformly and periodically arranged in the main scanning direction are used. As a result, similarly to the center of the light guide, the light beams are diffused at its edges in the main scanning direction, and hence a large amount of light beams is diffused also in a region other than a predetermined region to be illuminated. Therefore, it is required to improve the light use efficiency of the light beams emitted from the light sources. In a case where the multiple light sources are arrayed at irregular intervals, it is further required to suppress the illuminance unevenness in the main scanning direction.
In the case where the multiple light sources are arrayed at irregular intervals, at the edges in the main scanning direction where the light sources are densely arrayed so as to ensure sufficient peripheral light intensity, the degree of light diffusion may be lower than at the center where the light sources are arrayed sparsely. Further, it is important to suppress reduction in efficiency due to the light diffusion beyond the reading range. On the other hand, at the center in the light source array direction (main scanning direction), the light sources are arrayed sparsely, and therefore a diffusion surface having a higher efficiency of diffusion is necessary to reduce local fluctuation in light intensity.